1. Field of the Invention
The present invention concerns high-frequency data transmission cables and a method and apparatus for fabricating a cable of this kind.
2. Description of the Prior Art
Because computer and other networks use ever increasing data bit rates, it is necessary to improve the performance of cables. Two parameters in particular are very important in high-frequency data transmission: crosstalk between the pairs of conductors and the regularity of the impedance of these pairs.
Crosstalk between pairs is a phenomenon whereby signals transmitted in one pair generate noise in the adjacent pairs. Other things being equal, crosstalk is reduced by increasing the distance between the pairs. The regularity of the impedance of a pair is determined by a reflection measurement; it can be obtained easily in a twisted pair of the conductors which have an insulation of constant thickness and good concentricity, but is more difficult to obtain in the case of quads, because there is a risk of the conductors being locally at a greater distance from each other.
Although the invention applies essentially to cables having two to eight groups of twisted conductors, each group preferably comprising two or four twisted conductors, it applies to other numbers of conductors. Sheathed electrical cables comprising a plurality of conductors and a core filled with a plastics material foam are known in themselves. For example, U.S. Pat. No. 3,681,510 describes a sheathed electrical cable having a plurality of conductors and the core of which is filled with a plastics material foam. Insulated conductors are uniformly distributed in the core, both at the center and towards the periphery, which is delimited by a material that is also coated with the plastics material foam. In this cable the pairs of conductors are not individualized, and this cable is therefore not suited to high-frequency data transmission.
Japanese patent application JP-52 82 921 describes a cable formed by accommodating a flat cable wound on itself, comprising some ten parallel wires, for example, so that it fits in a circular section sheath, the interior space being filled with polyethylene foam. In cross-section the disposition of the conductors is substantially in the form of a spiral, one edge conductor of the flat cable being placed towards the interior of the circular cable and that at the other edge being in contact with the sheath. A small number of conductors is placed at the outside periphery and all the conductors are adjacent to each other. Crosstalk between pairs is increased relative to that of the flat cable on its own, because the edge conductors are also near the other conductors.
U.S. Pat. No. 4,755,629 discloses a cable for use in local area networks in which twisted pairs are each placed in a sheath. The pairs for transmitting data are surrounded by a shield. Other pairs, on the outside, are intended to transmit voice signals and not data. According to this patent, to reduce crosstalk the twisted pairs are moved apart by incorporating each pair into a circular section sheath, the two sheaths being in contact but holding the pairs apart.
The invention concerns a different construction enabling crosstalk to be reduced by moving the pairs apart and procuring excellent regularity of impedance.
In accordance with the invention, the groups of twisted conductors, which are preferably pairs or quads, are at a maximum distance from each other because they are placed in a member which, during its construction, separates them as far as possible, within the limits of a mold that forms the outside surface of the inside part of the cable. In this way the groups of twisted conductors which are fed in so that they are equidistant at the inside periphery of the mold are separated as far as possible, i.e. pushed against the inside surface of the mold, by the foam insulative internal member that holds them in place. In this way the pairs are as far apart as possible, given the imposed diameter, and this reduces crosstalk. The pairs are positioned by the foam when it has hardened, so that each twisted conductor group retains its regularity of impedance.